Scientists have developed a new way of testing whether a lethal form of malaria is resistant to potent artemisinin drugs, according to a study published in The Lancet Infectious Diseases.
Researchers from Cambodia and the US have created simple in-vitro and ex-vivo ring-stage survival assays (RSAs), which they say can clearly identify artemisinin-resistant, slow-clearing Plasmodium falciparum (P falciparum) parasites in those who have malaria.
According to the World Health Organization (WHO), there were around 219 million cases of malaria worldwide in 2010 and an estimated 660,000 deaths from the disease.
Efforts to combat the spread of malaria are now mainly dependent on artemisinin-based control treatments (ACTs), as the malaria parasite became resistant to older drugs that treated the disease.
But there is concern that if these ACTs also develop resistance, there are no other drugs available for widespread use.
Standard laboratory artemisinin susceptibility tests have yet to determine the difference between slow parasite clearance, which is found in those who are infected with drug-resistant parasites, and fast parasite clearance.
For the study, the research team set out to determine whether a novel in-vitro RSA could distinguish this difference.
They also wanted to analyze the in-vitro response to DHA (dihydroartemisinin) – the active metabolite of all artemisinins – within three separate blood stages of parasites (early rings, late rings and trophozoites), and see whether an ex-vivo RSA could detect P falciparum infections that are artemisinin resistant.
Artemisinin resistance ‘linked to early-stage malaria’
Using parasites from the Pursat Province – an area in western Cambodia that is artemisinin resistant – and parasites from Preah Vihear and Ratanakiri – artemisinin-resistant areas in northen and eastern Cambodia – the researchers exposed them to a “clinically-relevant pulse of DHA” (6-hour pulse of 700 nM).
When their survival was measured 72 hours later, early rings (defined as 0-3 hours after invasion) from slow-clearing parasites survived DHA significantly better, compared with those from fast-clearing parasites.
Late rings and trophozoites from slow-clearing and fast-clearing infections showed no susceptibility when exposed to DHA.
Didier Menard, of the Institut Pasteur du Cambodge in Cambodia and lead study author, says:
“We were able to clearly see the difference in the clinical response to artemisinins between people infected with parasites that were drug-resistant or drug-susceptible in vitro.
Our observations confirm that artemisinin resistance is associated with the very early stages of parasite development in the blood.”
Additionally, the findings showed that in the ex-vivo RSA, the survival rate of parasites significantly correlated with parasite clearance half-life – the time it takes for a drug to cut the number of parasites in the blood by half.
The test was also able to accurately detect slow-clearing infections in both the Preah Vihear and Ratanakiri areas of Cambodia where they have not yet been reported.
Test ‘could lead to better understanding of resistance’
Rick Fairhurst, of the US National Institutes of Health and co-leader of the study, explains that the in-vitro RSA can be used to better understand the molecular mechanisms of artemisinin resistance, to investigate the mode of action on artemisinins, and to screen and identify next-generation antimalarial drugs that can effectively kill artemisinin-resistant parasite strains.
“On the other hand,” he continues, “the ex-vivo RSA can be readily implemented in field-based settings to monitor the worsening of artemisinin resistance in Cambodia where it is highly prevalent and to map its spread to other regions of southeast Asia.”
“Also, this simple test can be easily established at sentinel sites in sub-Saharan Africa, where the arrival or evolution of artemisinin-resistant P falciparum is expected to be particularly devastating.”
Carol Hopkins Sibley of the WorldWide Antimalarial Resistance Network (WWARN) says in a linked comment to the study that these assays will allow fast validation of candidate molecular markers “by directly testing the correlation of proposed markers with the output from their survival assay.”
“The in-vitro test will also provide a platform for understanding the mechanism of the reduced artemisinin response,” she adds.
“With these simple methods in place, rapid tracking of the geographical and temporal changes in artemisinin resistance will be feasible in many sites. This far more comprehensive information will allow policy makers to design effective responses to the threat of artemisinin failure, and prolong the useful therapeutic life of ACTs.”
Medical News Today recently reported that an early-stage clinical trial of a vaccine against malaria called PfSPZ, was shown to be safe, generated an immune response and may provide 100% protection against the disease.
Written by Honor Whiteman
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Novel phenotypic assays for the detection of artemisinin-resistant Plasmodium falciparum malaria in Cambodia: in-vitro and ex-vivo drug-response studies, published in the The Lancet Infectious Diseases, 11 September 2013.
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